The present invention relates to motor drive technology, and more particularly, to a motor drive technology of a pulse width modulation (PWM) system.
FIG. 9 is a block diagram of a conventional motor driver and a motor driven with the motor driver. FIG. 2 is a graph showing target waveforms of phase currents for a motor 10. A position detection circuit 22 outputs signals corresponding to the position of a rotor of the motor 10 based on outputs of a Hall sensor circuit 21. A torque signal generation circuit 30 generates a signal TS of a sawtooth wave having a peak value corresponding to a torque instruction voltage TI and a period equal to the time period corresponding to an electrical angle of 60° of the motor 10. A logic control circuit 40 generates switching operation control signals F1 and F2 for defining the time period during which drive transistors 1 to 6 are put in the ON state. A phase switch circuit 23 turns ON the drive transistors 1 to 6 according to the signals output from the position detection circuit 22 and the switching operation control signals F1 and F2.
FIG. 10 is a graph showing phase currents for the motor 10 driven with the motor driver of FIG. 9 and other signals, in which periods TW2 and TV1 in FIG. 2 are shown in detail in an enlarged view. First, the period TW2 will be described.
With input of a reference pulse PI, two flipflops of the logic control circuit 40 are set, and the phase switch circuit 23 turns ON a U-phase upper arm side drive transistor 1, a V-phase upper arm side drive transistor 3 and a W-phase lower arm side drive transistor 6, for example (period T1). At this time, the sum of a U-phase current I1 flowing through a U-phase coil 11 and a V-phase current 12 flowing through a V-phase coil I2, that is, the magnitude of a W-phase current 13 flowing through a W-phase coil I3 can be detected with a current detection resistance 7. Flowing through the coil load, the current gradually increases with conduction of the drive transistors 1, 3 and 6. Once the voltage generated at the current detection resistance 7 reaches the torque instruction voltage TI with increase of the current flowing through this resistance, one of the flipflops of the logic control circuit 40 is reset with the output of a comparator 51, and this turns OFF only the drive transistor 1.
The drive transistors 3 and 6 are kept in the ON state. At this time, therefore, the magnitude of the current flowing through the V-phase coil 12 and the W-phase coil 13 can be detected with the current detection resistance 7. The current flowing through the V-phase coil 12 and the W-phase coil 13 continue increasing, and once the voltage generated at the current detection resistance 7 reaches the signal TS output from the torque signal generation circuit 30, the other flipflop of the logic control circuit 40 is reset with the output of a comparator 52, and this turns OFF the drive transistor 3.
The time period from the setting of a flipflop of the logic control circuit 40 until the reset thereof is an ON period of switching operation. After the reset of the flipflop, the currents flowing through the U-phase, V-phase and W-phase coils 11, 12 and 13 become regenerative currents passing through diodes existing between the source and drain of the drive transistors 2 and 4 in an attempt of maintaining the flowing state.
Since the regenerative currents do not pass through the current detection resistance 7, the voltage generated at the current detection resistance 7 is equal to a voltage generated with the V-phase current I2 during flow of a U-phase regenerative current (period T2), and it is zero during flow of U-phase and V-phase regenerative currents (period T3). The regenerative current gradually decreases. When the reference pulse PI is input again, the flipflops of the logic control circuit 40 are set. The drive transistors 1 and 3 are turned ON, and the operation described above is repeated.
FIG. 11 is an illustration of routes of currents during the period T3 in FIG. 10.
Referring to FIG. 11, the U-phase current I1 flows through a diode 2D, the U-phase coil 11, the W-phase coil 13 and the W-phase lower arm side transistor 6 as a regenerative current, and the V-phase current I2 flows through a diode 4D, the V-phase coil 12, the W-phase coil 13 and the W-phase lower arm side transistor 6 as a regenerative current.
As a result of the alternate flow of a drive current and a regenerative current by the switching described above, a motor phase current of a trapezoidal wave as shown in FIG. 2 having a peak value corresponding to the torque instruction voltage TI is allowed to flow to a predetermined coil load in synchronization with the output of the position detection circuit 22. Such a motor driver as that described above is disclosed in Japanese Laid-Open Patent Publication No. 2003-79182, for example.
The operation of the motor driver of FIG. 9 during the period TV1 in FIG. 10 will then be described. FIG. 12 is an illustration of routes of currents during a period T91 shown in FIG. 10. In the period T91, the V-phase current I2 flows through the V-phase upper arm side transistor 3, the V-phase coil 12, the W-phase coil 13, the W-phase lower arm side transistor 6 and the current detection resistance 7. If the U-phase current I1 has not sufficiently decreased by the start of the period T91, the U-phase current I1 continues flowing as a regenerative current through the U-phase lower arm side transistor 2, the U-phase coil 11, the W-phase coil 13 and the W-phase lower arm side transistor 6 even after conduction of the transistor 2.
In the case described above, both the V-phase current I2 and the U-phase current I1 flowing as a regenerative current flow through the W-phase coil 13. Because a regenerative current does not flow through the current detection resistance 7, only the V-phase current I2 flows through the current detection resistance 7 and increases to reach a target current of the magnitude corresponding to the torque instruction voltage TI, until the regenerative current becomes zero. As a result, a current greater than the current determined by the torque instruction voltage TI by the magnitude of the regenerative current will flow through the W-phase coil 13.
As described above, in the motor driver of FIG. 9, in the case that the load of the motor is large, for example, the current of a phase for which the current should be increased finds difficulty in increasing, while the current of a phase for which the current should be decreased finds difficulty in decreasing, due to influence of an induced voltage and the like. Also, when the time period corresponding to the electrical angle 60° is short, as during high-speed rotation of a motor, the ratio of the switching period to this time period is great.
As a result, the phase of the phase current supplied to the motor delays with respect to a position signal PS indicating the position of the rotor, causing a problem that the current of a phase for which the current should be decreased fails to decrease to zero within the time period corresponding to the electrical angle 60°. This disadvantageously generates brake torque on the motor and thus degrades the efficiency of the motor.
Moreover, if the current of a phase for which the current should be decreased fails to decrease to zero within the time period corresponding to the electrical angle 60°, a phase current greater than the current determined by the torque instruction voltage TI will flow for a certain duration during the time period corresponding to the next electrical angle 60°. This may result in any of the drive transistors 1 to 6 receiving flow of a current of a magnitude exceeding its absolute maximum rating.